1. Field of the Invention
The present invention relates to a battery comprising a battery element which includes a cathode, an anode and an electrolyte and is accommodated in a film-shaped casing.
2. Description of the Related Art
In recent years, the development of cordless and portable electronic devices such as notebook personal computers and cellular phones have been advanced, so low-profile, compact and lightweight electronic devices have been developed one after another. Moreover, recently diversification of electronic devices causes an increase in power consumption, so higher capacity of batteries as energy sources of the electronic devices, specifically higher capacity of secondary batteries has been in increasing demand.
Secondary batteries conventionally used include lead-acid batteries, nickel-cadmium batteries and the like, and as new secondary batteries, nickel-hydrogen batteries and lithium-ion batteries have been put to practical use. However, these secondary batteries use a liquid as an electrolyte, so a problem of liquid leaks from the batteries arises.
Therefore, in order to overcome the problem, a polymer lithium-ion secondary battery using a gel electrolyte formed through expanding a high molecular weight compound by using an electrolyte solution or holding an electrolyte solution in a high molecular weight compound has been developed. There is a polymer lithium-ion secondary battery in which a battery element is interposed in a laminate film used as a casing, and edge portions of the casing are bonded together by thermally fusion bonding to seal the battery element (for example, refer to Japanese Unexamined Patent Application Publication No. Hei 3-62447). According to the secondary battery, a further reduction in size, weight and profile can be achieved, and its energy density can be improved.
However, water easily enters into the laminate film, compared to a metal container (battery can), so in the battery, lithium-ion consumption or generation of a decomposed gas, hydrofluoric acid or the like by decomposition of water occurs, thereby a decline in the capacity of the battery is apt to increase.
In order to reduce the entry of water, it is effective to secure a certain depth (sealing width) of a thermally fusion bonded portion. For example, in a battery with as small a size as the battery can be mounted in a cellular phone, the sealing width is approximately 5 mm in general. However, under the condition, it is difficult to improve the energy density of the battery. Therefore, a contrivance such as folding the thermally fusion bonded portion has been achieved; however, it is difficult to apply the contrivance to a portion where a cathode terminal or an anode terminal is derived in the thermally fusion bonded portion